An effective blood stage Plasmodium falciparum malaria vaccine would be of immense global health benefit, particularly for infants and young children who suffer the greatest burden of malaria morbidity and mortality. We propose to use novel molecular epidemiological and structural vaccinology approaches to understand the human immune response to and the resulting antigenic diversity of apical membrane antigen-1 (AMA1), a leading malaria vaccine candidate antigen. AMA1 is highly polymorphic; antigenic variation is likely a parasite immune escape mechanism that allows repeated infection with different variants in the face of natural immunity. We recently completed the first Phase 2 clinical trial of a monovalent AMA1- based malaria vaccine in children in Mali, West Africa. The vaccine had limited overall efficacy but good allele-specific efficacy against clinical malaria caused by parasites with AMA1 homologous to the vaccine strain with respect to immunologically relevant polymorphisms in AMA1. Molecular epidemiological and animal studies suggest that some regions of the AMA1 molecule represent immunogenicity hot spots, i.e. residues within these regions are critical to development of strain-specific protective immunity. The combined results of these clinical and laboratory studies suggest that the vast diversity of AMA1 alleles may be consolidated into a smaller number of subgroups, thereby reducing the number of variants that would have to be included to achieve a broadly cross-protective AMA1 vaccine. The proposed translational research project will employ novel molecular epidemiological approaches that we have developed, as well as a novel autotransporter protein expression system, to analyze patient serum samples collected in prospective longitudinal studies in a malaria endemic area, with the aim of identifying cross-reactive and cross-protective AMA1 epitopes.